The present invention relates to ring laser angular rate sensors in general and, more particularly, to a dithering system for such devices. Further, the present invention also relates to any spring mass system where controlled back and forth rotation is of importance.
A bothersome characteristic of the ring laser angular rate sensor is the phenomenon known as "lock-in". At sensor rotation rates below a critical value called the lock-in threshold or lock-in rate the frequency difference between the oppositely traveling laser beams tend to synchronize to a common value resulting in the frequency difference going to zero indicating no rotation at all. The lock-in characteristic arises due to mutual coupling between oppositely traveling beams in the sensor. The dominant source of the coupling is mutual scattering of energy from each of the beams into the direction of the other. The effect is similar to lock-in coupling effects which have been long understood in conventional electronic oscillators.
One technique for attempting to reduce or eliminate the effects of lock-in is to mechanically dither or oscillate the laser angular rate sensor, back and forth, so that the laser angular rate sensor is operating above the lock-in threshold for a majority of the time. The system using this principle is disclosed in U.S. Pat. No. 3,373,650, by Joseph E. Killpatrick, entitled "Laser Angular Rate Sensor", and assigned to the same assignee as the present application. Disclosed in U.S. Pat. No. 3,373,650 is a dither system for oscillating a spring mass system including the laser angular rate sensor. The dither system described therein will oscillate the laser angular rate sensor at the natural resonant frequency of the spring mass system and will oscillate, back and forth, at substantially a constant peak-to-peak dither angle amplitude.
Recent developments in ring laser angular rate sensor technology have found it desirable to control the dithering motion of the laser angular rate sensor in a more controlled manner. It is the object of the invention to provide a dither system for oscillating the laser angular rate sensor having controlled clockwise and controlled counterclockwise dither angle amplitudes in response to a dither angle amplitude control signal.